[MD Bookmarks] Fix segfault when calling chrome.bookmarks.remove().

chrome/browser/sync/glue/extensions_activity_monitor.cc

Index: chrome/browser/sync/glue/extensions_activity_monitor.cc
diff --git a/chrome/browser/sync/glue/extensions_activity_monitor.cc b/chrome/browser/sync/glue/extensions_activity_monitor.cc
index 60d2284a3915396162138052461cf0079aece541..bb48f56c3e06a586f464b1edfbef94cfb663e38d 100644
--- a/chrome/browser/sync/glue/extensions_activity_monitor.cc
+++ b/chrome/browser/sync/glue/extensions_activity_monitor.cc
@@ -47,6 +47,9 @@ void ExtensionsActivityMonitor::Observe(
   DCHECK_EQ(extensions::NOTIFICATION_EXTENSION_BOOKMARKS_API_INVOKED, type);
   const extensions::Extension* extension =
       content::Source<const extensions::Extension>(source).ptr();
+  if (!extension)
+    return;
+
   const extensions::BookmarksFunction* f =
       content::Details<const extensions::BookmarksFunction>(details).ptr();
   switch (f->histogram_value()) {